This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. Section 119 from an application for METHOD OF LOCATING A MOBILE PHONE BY MEASURING THE DISTANCE BETWEEN THE PHONE AND A BASE STATION IN A CELLULAR MOBILE TELEPHONE SYSTEM filed earlier in the Korean Industrial Property Office on the 23th day of January 1999 and there duly assigned Ser. No. 2064/1999.
1. Field of the Invention
The present invention relates to a cellular mobile telephone system (CMTS), and more particularly a method for providing more accurate measurement of the distance between a mobile phone and a base station by incorporating a round-trip delay (RTD) and the mean error distance therebetween.
2. Description of the Related Art
A geographic area served by the CMTS is divided into a group of smaller areas known as cells. Each cell includes a base station to provide communication with a mobile phone or a portable instrument within the boundary of a cell. In addition, all the base stations are controlled by a mobile switching center (MSC) to enable communication between the cells.
Referring to FIG. 1, a network of the CMTS using the CDMA (Code Division Multiple Access) technology generally consists of a plurality of base stations 20, 30, 40 for providing communication with mobile phones 10 and an MSC 50 for connecting the mobile phones with a public switched telephone network (PSTN) 60. This type of CMTS is used to provide additional service to the mobile phone users 20, 30, 40, by locating their positions in relation to the surrounding base stations. To this end, various methods have been developed to measure the distance between the mobile phone and the base station.
One method uses the time of arrival (TOA) measurement. Namely, a mobile phone transmits a prescribed message signal to at least 3 adjacent base stations, thus determining the distance between the mobile phone and each respective base station using the transmission time of the message signal. Calculating a radial distance between the mobile phone and three base stations provides an accurate fix, as the intersection of the three spheres. However, in this case, the mobile phone should be provided with the means for transmitting the prescribed message to the base stations.
Another method uses a round trip delay (RTD) which uses a prescribed message transmitted from the base station to the mobile phone and measures the time it takes to complete a round-trip for the prescribed message to return back to the base station. However, in this case, radio waves of the message are subjected to various microwave obstructions, such as buildings, thus the RTD inherently includes a delay resulting from reflection by such obstructions. This type of delay degrades the accuracy of locating the exact position of a mobile phone.
It is an object of the present invention to provide a method of obtaining a reliable measurement of the distance between a mobile phone and a base station based on the RTD measurement adjusted by a GPS system.
It is another object of the present invention to provide a method of calculating the mean error distance to adjust the distance between the mobile phone and the base station obtained by the RTD measurement.
According to an aspect of the present invention, a method of measuring the mean error distance to adjust the distance between a mobile phone and a base station obtained by a round-trip delay (RTD) measurement of a prescribed message transmitted from the base station to the mobile phone in a cellular mobile telephone system (CMTS), the method comprises the steps of determining a plurality of measuring points within the cell boundary of the base station to be used for obtaining the mean error distance; measuring the respective RTDs between the measuring points and the base station by sequentially positioning the mobile phone at each of the measuring points; calculating the respective RTD distances between the measuring points and the base station based on the respective RTDs; obtaining the GPS (Global Positioning System) data of the base station and the measuring points; calculating the respective GPS distances between the base station and the measuring points based on the GPS data; and, calculating the respective error distances based on the difference between the respective RTD distances and the GPS distances for all measuring points and obtaining the mean error distance from these respective error distances.
According to another aspect of the present invention, a method of measuring the distance between a mobile phone and a base station in the CMTS, comprises the steps of calculating a mean error distance to adjust the distance between the mobile phone and the base station within the cell boundary of the base station; measuring the RTD between the base station and the mobile phone positioned at an arbitrary point within the cell; calculating a new and more accurate RTD distance between the mobile phone and the base station based on the RTD measurement by subtracting the mean error distance from the measured RTD distance.